I just noticed this thread and thought I would make some comments because we see/hear about the Birds suspension a lot more often in the UK than I guess you do.

As you've rightly pointed out, Birds are just selling Bilstein equipment which has been modified to their specifications. The price is (in my opinion) very high for what you get, but what you're actually paying for is the knowledge, experience and research which Birds have put into the kit.

It was developed about 4-5 years ago now in conjunction with a member here called E92Fan (who used to run one of the most extensively modified E92 335i's in the UK) and the kit is specifically designed to meet and correct the flaws that the UK market found early on with the E92. Specifically, E92's all feature Sport suspension in the UK (whether SE or M-Sport spec) and run flat tyres. In conjunction with our poorly maintained (and generally bumpy/undulating) roads, the combination of rock hard suspension and steel reinforced tyres is extremely uncomfortable.

From a 'sports driving' kind of perspective, the UK's best roads are largely these kind of smaller back roads which suffer from a very poor road surface - and when M-Sport/Runflat equipped BMW's tackle these roads at speed, the result is that the standard suspension cannot react fast enough (and is too harsh) so the back end skips about a lot and does not allow for progressive power delivery. This is especially true when considering higher power cars.

So basically, the Birds kit should allow for a much better feel when driving on poor road surfaces, whilst at the same time reducing body roll AND in general providing a better ride than a stock car with sports suspension. You will want to ditch your run flat tyres if you have them too, as Birds have only really tested/reccomended the kit for use with premium sports tyres.

I've been in E92Fan's car with this setup both in the UK and on the Nurburgring and I've also been in another member here's car on the 'Ring with the same kit fitted. I think it's a very good, fixed suspension kit which will suit the needs of anyone who doesn't want a crazy drop (that only coilovers would achieve) and who wants to maximise comfort AND fast driving capability.